Polyamides are attractive materials to use in many demanding applications because of their mechanical properties and chemical resistance. Many of these applications involve use at high temperatures. For example, components used in the automotive engine compartment such as ducts, fans and fan shrouds, manifolds, tubes, etc. require operation at high temperatures. In all of these applications, it is desirable that the structures and components retain their mechanical properties such as stiffness, strength and creep resistance at high temperatures.
The change in properties of a semicrystalline polymer with temperature is governed by its glass transition temperature. This is a temperature characteristic of a polymer's molecular architecture, when molecules undergo a transition from a glassy state to a rubbery state. The mechanical properties such as stiffness and strength exhibited by a polymer in the glassy state are generally significantly higher than those in the rubbery state.
At high temperatures the rate of thermal oxidative degradation of a polymer part is higher, leading to loss of mechanical properties at a faster rate. The degradation causes surface embrittlement of any surface exposed to air. Surface embrittlement has a severe effect on the physical properties of the tubes and pipes since external bending and flexing loads typically give rise to high stress concentrations at the surface.
A common approach to retarding oxidative degradation of polyamides is to use thermal stabilizers, at low levels, dispersed throughout the polymer matrix. The thermal oxidation stabilizers used in polyamides generally fall into three groups: (i) organic stabilizers based on aromatic amines, (ii) organic stabilizers based on hindered phenols sometimes in combination with phosphorous based compounds, and (iii) inorganic stabilizers based on copper and halogen compounds. The organic stabilizers are often not suitable for incorporation into polyamides that need to be processed at temperatures close to 300° C. or higher, as they tend to volatilize or decompose. Copper based inorganic stabilizers are also not suitable because they lead to degradative reactions at these high temperatures. This is especially true in processes such as extrusion, blow molding, casting, film blowing etc. that involve exposure of the polymer melt to atmospheric conditions.
U.S. Pat. No. 5,219,003 discloses a tube with low-temperature impact resistance that is suitable for conveying motor vehicle engine fuel and that comprises three layers that are made from at least two mutually compatible polyamides. The inner and outer layers contain impact modifiers and the middle layer contains substantially none.
U.S. Pat. No. 7,122,255 discloses multi-layered polyamide composite articles comprising at least three layers that comprise aliphatic and semi-aromatic polyamides and that is better able to retain its mechanical properties at high temperatures and over long times are disclosed. Layers comprising aliphatic polyamides may optionally contain oxidation stabilizers. The articles may be in the form of tubes incorporated into a heat exchanger.
Tubes fabricated from polyamides that simultaneously offer improved retention of mechanical properties at high in-use temperatures and stabilization against thermal degradation are needed. Additionally tubes having a variation in composition and physical properties through the thickness of the tube may be useful and available by co-extrusion processing. However, the variation in thermal processing required for various polymer compositions having widely disparate thermal properties in co-extrusion processes lead to significant manufacturing problems.
The object of the present invention is to provide multi-layered co-extruded tube comprising a plurality of layers of polyamides and process for the manufacture of these articles such that the problems associated with the incorporation of commonly available thermal stabilizers into the polyamides, the retention of the tube's mechanical properties at high in-use temperatures, and the manufacturing issues of co-extruded tubes are simultaneously addressed.